Topic 4B: Diversity And Selection Flashcards
What number of chromosomes do normal body cells have
Diploid (2n)
2 of each chromosomes one from mum and one from dad
What do chromosomes pair up in
Homologous pairs
How many homologous pairs do humans have
23 pairs and 46 chromosomes
What are the 2 types of gametes
Egg and Sperm
What number of chromosomes do gametes have
Haploid (n)
Only one copy of each chromosomes
What’s the haploid number of chromosomes
23
What happens in sexual reproduction
Two gametes join together at fertilisation to form a zygote which divides and develops into a new organism
What happens at fertilisation
A haploid sperm fuses with haploid egg, making a cell with the normal diploid number of chromosomes
Half of these chromosomes are from father half from mother
During sexual reproduction any sperm can fertilise any egg, fertilisation is random
What does random fertilisation produce
Zygotes with different combinations of chromosomes to both parents
Mixing of genetic material in sexual reproduction increase’s genetic diversity within a species
What is meiosis
Type of cell division that takes place in the reproductive organs of multicellular, eukaryotic organisms
Cells that divide by meiosis are diploid to begin with them become haploid
What are the 6 steps of meiosis
1) Before meiosis starts the DNA unravels and replicates so there are 2 copies of each chromosomes called chromatids
2) The DNA condenses to from double armed chromosomes each made from 2 sited chromatids they are joined in the middle by a centromere
3) Meiosis I (first division) the chromosomes arrange themselves into homologous pairs
4) These homologous pairs are then separated halving the chromosome number
5) Meiosis II (second division) the pairs of chromatids that make up each chromosome are separated (centromere is divided)
6) Four haploid cells that are genetically different from each other are produced
What are the 2 main ways that genetic diversity is created
Crossing over and independent segregation
What happens when crossing over occurs
Homologous chromosomes come together twist around each other and bits of the chromosomes swap over
The chromatids still contain the same genes but different combination of the alleles
What happens when independent segregation occurs
Each homologous pair of chromosomes in your cells is made up of one from mam and one from dad
When pairs separate in meiosis it’s completely random meaning each cell has a different combination of chromosomes
This ‘shuffling’ creates genetic variation in any potential offspring
What is a gene mutation
Involves a change in the DNA base sequence of chromosomes
What are the 2 types of gene mutation
Substitution and deletion
What is a substitution mutation
One base substituted with another
What is a deletion mutation
One base is deleted
What are the effects of gene mutations
Degenerate nature of genetic code means that same AA are coded for by more than one DNA triplet
Meaning not all substitution mutations result in a change to the AA sequence of the protein
But deletions always will lead to changes in the AA sequence as it changes the number of bases present and leads to a frame shift
What are mutagenic agents
Mutations occur spontaneously but some things can cause an increased rate of mutations these are mutagenic agent
Give 2 examples of mutagenic agents
UV light and Ionisation radiation
What is a chromosome mutation
When Changes to the number of chromosomes occurs usually due to errors in meiosis
What do chromosomes mutations lead to
Inherited conditions
Give an example of chromosome mutation and explain it
Chromosome disjunction: Failure for chromosomes to separate properly
What is genetic diversity
Number of different alleles in a species or population
A large number of alleles in a population leads to a high genetic diversity
Why is a low genetic diversity in a population bad
Might not be able to adapt to change in the environment and the whole population could be wiped out by a single event (e.g diversity)
In what 2 ways can genetic diversity be increased by in a population
Mutations in the DNA forming new alleles, some of these can be advantageous whilst others can lead to problems
Different alleles being introduced into a population when individuals from another population migrate into it anD reproduce (this is known as gene flow)
What allows for natural selection
Genetic diversity
What is a genetic bottle neck
Event that causes a big reduction in the population
This reduces the number of different alleles in the gene pool and so reduces genetic diversity
The survivors reproduce and larger population is created from a few individuals
What does the founders effect describe
What happens when just a few organisms from a population start a colony and there’s only a few small number of different alleles in the initial gene pool. The frequency of each allele in the new colony might be very different to that in the original population
What can the founders effect occur as a result of
Migration leading to geographical separation
What is natural selection
Allele cod’s for a characteristic that increases the chances of an organism serving its frequency in the pop then increases
Whats the 5 steps to natural selection
1) Not all individuals are as likely to reproduce as each other (differential reproductive success) a random mutation may occur creating an advantageous allele to be produced which is beneficial
2) meaning they are more likely to survive and reproduce passing allele onto offspring
3) those without it are less likely to survive and causes them to eventually die off leaving only those with beneficial allele
4) so frequency of this allele increases in each generation
5) over generations it leads to evolution as advantageous allele becomes common in the pop
What is evolution
Gradual change in species overtime, it has led to the huge diversity of living organisms
What are the 3types of adaptation
Behavioural, physiological and anatomical (structural)
What are behavioural adaptations
Ways organisms act that increase its chance of survival
What are physiological adaptations
Processes inside the body that increase chance of survival
What are anatomical adaptations
Structural features of an organisms body that increase chance of survival
What are the two types of selection
Stabilising and directional
What is directional selection
Where individuals with alleles for characteristics of an extreme type are more likely to survive and reproduce, this could be in response to an environmental change
What is stabilising selection
Where individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce, occurring in response to an environmental change and reduces the range of possible characteristics
